This invention relates to a process for the upgrading or beneficiation of oil shale and is particularly concerned with upgrading raw oil shale by physically removing a substantial portion of the lower grade shale particles.
Oil shale is a sedimentary rock having a predominately fine-grain structure containing a hydrocarbon material referred to as kerogen, a solid organic material of a high molecular weight which is insoluble in conventional organic solvents. Normally, oil shale will contain between about 5 and about 35 weight percent kerogen. Upon heating, a portion of the kerogen decomposes to produce a hydrocarbon liquid and gaseous product. The liquid hydrocarbon product is referred to as shale oil. Normally, the kerogen is recovered in the form of shale oil from oil shale by pyrolyzing or retorting the shale at high temperatures, generally temperatures between 900.degree. F. and 1100.degree. F. Since the kerogen content of oil shale is relatively low, large amounts of the shale must be retorted in order to recover significant amounts of shale oil. When the shale is subjected to such high temperatures it leaves behind an ash-like residue containing residual carbon which must be disposed of. Beacuse of the low yield of shale oil and the high yield of residue, it is desirable to upgrade the oil shale prior to pyrolysis or retorting by removing the lower grade shale particles which contain only small quantities of kerogen.
The particle density of oil shale is inversely proportional to the amount of kerogen content and it has been suggested that oil shale can be separated into low grade and high grade fractions by the use of the sink/float technique, which is commonly referred to as a gravimetric separation. The resultant low density fraction will be comprised of high grade particles of shale which will contain substantially more kerogen than the particles comprising the low grade fraction and therefore can be selectively processed for the recovery of the kerogen in the form of shale oil. Although such an upgrading process will increase the yield of oil shale and decrease the amount of residue produced during retorting, there is still room for substantial improvement in upgrading the oil shale to increase the yield of liquid hydrocarbons and decrease the production of inorganic residue materials.